Objective  The aim of this experiment was to study the effects of high starch and oil diet on milk performance and mammary gland transcriptome of dairy cows, and explore the molecular mechanism of milk fat depression (MFD).

Method Eight lactation Holstein cows were randomly divided into two groups of the control group and treatment group, four cows in each group with a 2×2 crossover experiment. The test was divided into two periods, each period was 23 days, and two groups were switched in the second period. In the first 16 days of each period, the cows in the control group were fed low starch and oil diet, the net lactation energy of the control diet was 6.78 MJ/kg. For the cows in the treatment group, 266 g/kg fine ground corn and 46 g/kg soybean oil (dry matter basis) were added to the control diet. In the last seven days of each trial period, the cows of two groups were both fed with low starch and oil diet, the net lactation energy of the diet in the treatment group was 7.66 MJ/kg. Milk yield and milk composition were measured on the 1st, 4th, 7th, 10th, 13th, 16th, 19th and 22nd days of each period respectively, and mammary tissues were collected on the 16th day to detect changes in transcripts.

Result Compared with the cows fed with low starch and oil diet, the dry matter intake and milk yield of cows fed high starch and oil diet decreased on the 13th and 16th days (P<0.05). After feeding high starch and oil diet for seven days, the milk fat percentage began to decrease, and the milk fat rate and milk fat yield decreased significantly on the 10th, 13th, 16th and 19th days (P<0.05). The contents of milk protein, lactose and non-fat solids increased significantly on the 13th, 16th and 19th days (P<0.05). A total of 235 differentially expressed genes (DEGs) were detected in the control and treatment groups, 64 of which were up-regulated and 171 were down regulated. GO analysis showed that the DEGs were mainly related to inflammatory reaction, α-amino acid metabolism, organic nitrogen compound metabolism, cell development, lipid biosynthesis and adipocyte differentiation. KEGG analysis showed that the DEGs were mainly related to axon guidance, NF-κB signaling pathway, calcium signaling pathway and so on. Combined with the correlation analysis between the DEGs and milk performance, this study screened some genes related to lipid regulation as candidate genes for studying lipid metabolism in dairy cows under MFD, including FGFR4, VDR, HTR2B, CCL21 and TYRP1 which were down regulated in mammary glands.

Conclusion Feeding cows with high starch and oil diet can reduce dry matter intake, milk yield and milk fat rate, increase the contents of protein, lactose and non-fat solids, and down-regulate the expression of fatty acid synthesis related genes in mammary glands of dairy cows. The study can provide data references for scientific formulation of diets and investigation of the molecular mechanism of MFD in dairy cows.